Method of and apparatus for regulating a forced flow steam generator



Jan. 5, 1965 F. LAUBLI 3, METHOD OF AND APPARATUS FOR REGULATING A FORCED FLOW STEAM GENERATOR Filed Feb. 12, 1962 5 Sheets-Sheet 1 FFP/ TZ Lfi Z/B I.

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Jan. 5, 1965 Filed Feb. 12, 1962 Jan. 5, 1965 F. LAUBLI 3,164,136

' METHOD OF AND APPARATUS FOR REGULATING A FORCED FLOW STEAM GENERATOR Filei Feb. 12, 1962 5 Sheets-Sheet s Inge/lion Jan. 5, 1965 F. LAUBLI 3,164,136 METHOD OF AND APPARATUS FOR REGULATING A FORCED FLOW STEAM GENERATOR Filed Feb. 12, 1962 5 Sheets-Sheet 4 Jul gator. F P/ rz L fiUEL Jan. 5, 1965 F. LAUBLI 3,164,13fi

METHOD OF AND APPARATUS FOR REGULATING A FORCED FLOW STEAM GENERATOR Filed Feb. 12, 1962 6 Sheets-Sheet 5 Fly. 5 119 g 1/7 EP/ 77 Anus-1.x 13V 144.

United States Patent 3,164,136 METHGD OF AND APPARATUS FQR REGULA IN'G A FSRCED FLOW dTEAh/l GENERATGR Fritz L'aluhli, Winterthur, Switzerland, assignor to Suizer Freres, S.A., Winterthur, Switzerland, a corporation of Switzerland Filed Feb. 12, 1962, Ser. No. 172,449 Claims priority, application Switzeriand, Feb. 15, 1961, 1,992/61 8 Claims. (Cl. 1Z2-47} The present invention relates to a method of and an apparatus for controlling the feedwater supply, the fuel supply and the combustion air supply to a forced flow steam generator in response to a main control signal corresponding to a temperature of the operating medium passing through the steam generator and in response to an auxiliary signal corresponding to a temperature of the operating medium upstream, with respect to the flow of the operating medium, of the locality to whose temperature the main control signal corresponds.

In a conventional method and apparatus of the aforesaid type the main control signal corresponds to the temperature of the steam after completion of the evaporation and the auxiliary control signal corresponds to the temperature of the operating medium entering the evaporating section of the forced flow steam generator. Each signal controls the operation of a feedwater valve, the valves being individually interposed in two feed pipes which are arranged in parallel relation with respect to the feedwater flow and which are connected downstream of the valves and upstream of the economizer to form a single conduit terminating in the economizer. The conventional method controls the mean value of the two temperatures. This control is not satisfactory because the two temperatures may be quite diiferent although their mean value does not change. A relatively great change of the enthalpy has little etfect on the temperatures on which the conventional method is based because these temperatures are measured close to the locality where the liquid changes into vapor. In the conventional method both control signals have a proportional characteristic and the control effected by the signals is subject to considerable control deviations which hinder the maintaining of constant temperatures.

It is an object of the present invention to provide a control system for a forced flow steam generator whereby the feedwater, fuel and combustion air supply are controlled quickly and accurately in response to the temperatures of the operating medium at two different locations in the steam generator.

It is a further object of the invention to provide a system for controlling the feedwater, fuel and combustion air supply to a forced flow steam generator substantially instantaneously in response to the temperature of the operating medium passing through the steam generator, preferably to the temperature in/ or close to the evaporating section of the steam generator, and for controlling the feedwater, fuel and combustion air supply relatively slowly but more accurately in response to the temperature of the superheated steam produced by the steam generator.

In the system acording to the invention a main control signal is produced which corresponds to the temperature of the steam passing through or leaving the final superheater of the steam generator, and an auxiliary control signal is produced which corresponds to the temperature of the operating medium at a point upstream of the final superheater, for example in the evaporating section of the steam generator. The main signal is transformed to a signal having a proportional-integral-differential (PID) character and the auxiliary signal is transformed to a signal having a proportional-differential (PD) character. The transformed signals are combined and the resulting combined signal is used for controlling the feedwater supply valve. The main signal is separately transformed to a signal having a PID character and the auxiliary signal is separately transformed to a signal having a PD character. The separately transformed signals are combined and the resulting signal is used for controlling the fuel and combustion air supply to the steam generator. In this way a rough control is effected without appreciable time lag by the auxiliary control signal and an accurate control is effected by the main control signal.

The feedwater valve and the apparatus for controlling the fuel and combustion air supply may be supplementally controlled by a control signal corresponding to the pressure of the live steam which signal is separately transformed to two signals, each having a PID character; one of the transformed pressure signals is combined with the combined main and auxiliary temperature responsive signal for supplementally controlling the feedwater valve. The second transformed pressure responsive signal is combined with the combined main and auxiliary temperature responsive signal for controlling the fuel and combussion air supply.

The combustion air supply may be supplementally controlled in response to the percentile CO content of the combustion gases.

In a modification of the invention the steam temperature may be initially used to control the injection of a coolant into the steam passing through the final superheater whereby the control signal responsive to the steam temperature is transformed to a signal having a PID character for controlling the coolant supply. A control signal is produced corresponding to the rate of flow of coolant to the superheater and used as the main signal in the aforedescribed control of the feedwater, fuel and combustion air supply. In a modification of this system the signal responsive to the rate of coolant supply may be transformed to a signal having an integral character and combined with the main signal of the first described control system according to the invention for controlling, in cooperation with the auxiliary signal, the feedwater, fuel and combustion air supply.

In a further modification, the main signal is transformed to a signal having a proportional-differential signal and used for controlling the injection of a coolant into the superheater, the main signal being also used for controlling the feedwater, fuel and air supply to the steam generator in the manner of the first described system according to the invention.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, and additional objects and advantages thereof will best be understood from the following description of embodiments thereof when read in connection with the accompanying drawing, wherein:

FIG. 1 is a diagrammatic illustration of a critical pressuwe forced flow steam generator and of regulating means according to the invention.

FIG. 2 is a part sectional diagrammatic illustration of the structure of portions of the apparatus which are schematically shown in FIG. 1.

FIG. 3 is a diagrammatic illustration of a critical pressure forced flow steam generator and of modified regulating means.

FIG. 4 is a diagrammatic illustration of a steam power plant including a forced flow critical pressure steam generator and of control means according to the invention.

FIGS. 5 to 7 diagrammatically illustrate portions of regulating systems for forced flow critical pressure steam generators according to the invention.

FIG. 1 shows a forced flow steam generator comprising an economizer 1, a section 2 wherein the water heated in the economizer 1 is converted into steam and a superheater 3 which may be composed of a plurality of serially arranged sections. The economizer 1 receives liquid operating fluid from a feed pipe 4 wherein a valve 5 is interposed which is actuated by a servomotor 6. The steam generator operates under critical pressure and the liquid operating medium changes into steam in the evaporating section 2 without ebullition and without the simultaneous presence of two phases of state of the fluid. The steam superheated in the superheater 3 flows through a steam main 7 to a steam consumer, for example a turbine, not shown. The steam generator comprises a combustion apparatus 8 receiving fuel through a conduit 9 and combustion air through a conduit 10. The conduits 9 and it) are controlled by regulating devices 11 and 12, respectively, which are actuated by servomotors 13 and 14, respectively.

A temperature measuring device 15 is connected to the outlet of the superheater 3 and produces a main temperature signal t which is conducted through a signal conduit 16 to a regulator 17 and also to a regulator 13. Both regulators have a proportional-integral-difterential character (PID). The outputs of the regulators 17 and 18 are conduted through conduits 19 and 2 respectively, to summing devices 21 and 22, respectively. A temperature sensitive device 23 is connected to the evaporator 2 and produces an auxiliary temperature signal t which is conducted through conduits 24 to a regulator 25 and to a regulator 26, both regulators having a proportional-differential characteristic (PD). The outputs of the regulators 25 and 26 are conducted through conduits 27 and 28, respectively, to the summing devices 21 and 22, respectively. In a steam generator which operates at subcritical pressures the temperature t would have to be measured either at the outlet of the economiz'er 1 or at the inlet of the superheater 3, because in a sub-critical pressure steam generator the temperature in the evaporator 2 is constant.

A signal conduit 29 conducts the signal produced by the summing device 21 to a summing device 30 which produces a signal conducted through a conduit 31 to the servomotor 6 for controlling the latter. A signal conduit 32 conducts the signal produced by the summing device 22 to a summing device 33 whose output is conducted through a signal conduit 34 to the servomotor 13 and to a summing device 35 whose output signal actuatcs the servomotor 14.

A pressure sensitive device 36 is connected to the steam main 7 and produces signals which are conducted through a conduit 37 to a regulator 38 and to a regulator 39. The regulators 38 and 39 have a proportionsl-integral-difierential character (PID) and their outputs are conducted through a conduit 4-!) to the summing device 33 and through a signal conduit 41 to the summing device 35 respectively. A percentile CO measuring device 43 is connected to the flue, not shown, of the steam generator and produces signals which are conducted through a signal conduit 42 to the summing device 35.

In the system illustrated in FIG. 1 temperature changes of the operating medium are measuredby the devices 23 and 15 and control regulators 25, 26 and 17, 18, respectively, which control the feedwater supply to the steam generator by actuating the feed valve 5 and also control the combustion apparatus by adjusting the regulating devices 11 and 12. The temperature measuring device 15 with the regulators 1'7 and 18 tends to maintain a constant steam temperature. The temperature measuring device 23 with the regulators 25 and 26 discovers irregularities sooner than the measuring device 15 and minimizes temperature changes at the outlet of the superheatcr 3 by relatively quickly adjusting the regulating devices 5, 11 and 12. In the same manner as the steam temperature is used for controlling the feedwater supply, the fuel supply and the'combustion air' supply, the pressure or the live steam measured at $6 is used to control through regulators 38 and 39 the feedwater supply, the fuel supply and the combustion air supply. The combustion air supply is additionally controlled according to the CO content of the flue gases for maintaining a predetermined CO content.

In a modification of the invention complete control circuits are provided in lieu of the servomotors 6, 13 and 14 whereby the rate of flow of feedwater is measured between the valve 4 and the economizer I and the rates of supply of fuel and combustion air are measured between the combustion apparatus 8 and the respective regulating devices, the measured values being compared with the output signals of the summing devices 39, 33 and 35, and the regulating devices 5, 11 and 12 being adjusted according to the deviation from predetermined values required to produce the desired temperatures.

In lieu of the signals produced by the pressure responsive device 36 a ditterent signal may be fed into the signal conduit 37, for example, a signal which is produced at a central control station and which corresponds to the desired output of the steam generator, or a signal which corresponds to the output of the prime mover driven by the steam produced in the steam generator, or a signal which is derived from a predetermined timetable for operating the steam generator, or a signal which corresponds to the frequency of an electric distributing system fed by a generator driven by a turbine operated by steam produced in the steam generator, or a signal corresponding to the speed of rotation of the turbine.

FIG. 2 more elaborately illustrates mechanical and hydraulic devices for implementing the system diagrammatically shown in FIG. 1. The device 15 for measuring the temperature of the steam at the outlet of the superheater 3 includes a rod 50 which has a relatively small temperature expansion coefiicient and is made, for example, of invar. One end of the rod St is connected to the outlet portion of a tube of the superheater 3, the other end of the rod being connected to a two-arm lever 51 which is pivotaliy connected to the tube 3 and whose free end is connected through a rod 52 and a spring 53 to a piston 54 movable in a cylinder having ports controlled by the piston 5d for admitting to and relieving a pressure fluid from the cylinder. The pressure prevailing in the cylinder, which pressure depends on the position of the piston 54, is conduced through the conduit 16 to the PID regulator 17. The latter comprises a cylinder 56 which is connected to the conduit 16 and contains a piston 58 movable in the cylinder between two springs 57 and being connected by a rod 59 to one end of a two-arm lever 60. A pilot valve 62 is connected to the lever by means of a rod 61, the pilot valve controlling the flow of a pressure fluid through pipes 63 to a hydraulic servomotor cylinder containing a piston 64. One side of the latter is connected by a piston rod to one end of a two-arm lever 65 which has a fulcrum connected to one end of a twoarm lever 66. The other side of the piston 64 is connected through a rod 67 to a cylinder 55' containing a piston 59 connected to a spring 68. Also connected to the spring 68 is the second arm of the aforementioned lever 60. The spaces in the cylinder 55' at opposite sides of the piston 69 are connected by a by-pass pipe 70 controlled by a valve 71. A pipe 72 is connected to the conduit 16 which pipe terminates in a cylinder '73 containing a piston 74 resting on a spring 81. The space below the piston 74, wherein the spring 81 is located, communicates with the atmosphere. The piston 74 is connected through a piston rod '75 to a piston '76 movable in a cylinder 77. The latter is supported between two springs 73 and the spaces at the opposite sides of the piston 76 in the cylinder 77 are connected by a by-pass pipe 79 controlled by a throttle 79'. The cylinder '77 is connected by means of a rod 80 to the second end of the twoarm lever 65.

The temperature measuring device 23 which is connected to the evaporator 2 is built like the measuring device 15. The output signal of the device 23 is conducted through the conduit 24 to the PD regulator 25. The latter has two cylinders 82 and 83 which are both connected to the conduit 24. The cylinder 83 is like the cylinder 73 of the PID regulator 17. The space below a piston 89 in the cylinder 82 is connected to a pressure fluid or signal conduit 49 through which a set point signal is supplied. Similarly, a set point signal is supplied to the cylinder 56 of the regulator 17 through a conduit 48. The piston 84 in the cylinder 83 of the regulator is connected to a piston 85 in a cylinder 86 held between two springs 93. The spaces in the cylinder 86 at opposite sides of the piston 85 are connected by a conduit 98 controlled by a valve 99. The device 85, 86, 93, 98, 99 corresponds to the device 76 to 79 of the regulator 17. A rod 87 connects the cylinder 86 to one end of a twoarm lever 88 which corresponds to the lever 65 in the regulator 17 and whose fulcrum is connected to the second end of the lever 66. The piston 89 of the cylinder 82 is connected by means of a rod 90 to the second end of the lever 88.

The lever 66, which constitutes the summing device 21 of FIG. 1, is supported by a spring 91 resting on a bell-shaped piston 92 moving in a cylinder whose interior is connected by the conduit 29 to a servomotor cylinder containing a piston 94 which is connected to one end of a two-arm lever 95. The piston 92 controls supply of a pressure fluid to and relieve from the cylinder wherein the piston 92 is movable. is connected through a link 97 to the valve 5 in the feedwater pipe 4. The second end of the lever 95 is connected to a servo-piston 96 moving in a cylinder whose interior is connected by the signal conduit 41 to the PID regulator 39 shown in FIG. 1. The PID regulator 39 is constructed like the regulator 17, the input signal of the regulator 39 corresponding to the pressure of the live steam in the steam main 7.

FIG. 2 shows a pipe connected to the conduit 24 and a pipe connected to the conduit 16. These pipes are connected to the regulators 26 and 18, respectively, shown in FIG. 1, which are constructed in the same manner as the regulators 17 and 25, the output of the regulators 18 and 26 being connected to the regulating devices 11 and 12 of the combustion apparatus 8.

An increase of the temperature of the evaporating section 2 or the steam generator causes clockwise movement of the level 51' and an increase of the pressure in the conduit 24 which acts on the pistons 89 and 84. The increased pressure moves the piston 89 and also the levers 88 and 66 downward so that the piston 92 is moved to increase the pressure in the conduit 29 which acts on the piston 94. The latter, through the linkage 95, 97 moves the valve 5 in counterclockwise direction so that more feedwater is supplied to the steam generator. The downward movement of the lever 88 is initially augmented by the pressure acting on the piston 84 because the system 85, 86 is initially rigid. Soon after, however, the rod 87 is moved upward by the pressure of the spring 93 so that fluid is forced from the space below the piston 85 through the by-pass conduit 98 into the space above the piston 35. The PD regulator 25 effects a rough control of the feedwater supply. The differential character of the control can be regulated by adjusting the throttle valve 99 in theby-pass pipe 98. The proportional character of the control can be altered by adjusting or exchanging the springs in the cylinders 82 and 83.

Fine adjustment is effected by the PID regulator 17. A rising temperature of the superheater 3 moves the lever 51 of the temperature measuring device 15 in clockwise direction whereby the pressure in the conduit 16 is in creased. This causes an upward movement of the piston 58 and a counterclockwise swinging of the lever 60 around the fulcrum which is suspended by the spring 68 so that also the pilot valve 62 moves upward. Pressure fluid is,

The fulcrum of the lever 95 therefore, supplied to the space above the piston 64 which moves downward and, in turn, moves the levers 65 and 66 downward. This efi ects an increase of the pressure below the piston 9.2 and opening of the valve 5. Downward movement of the piston 64 initially causes also a downward movement of the piston and of the piston 69 and slight downward movement of the lever and of the pilot valve 62. Thereupon the spring 68 pulls the left end of the lever 60 back into its initial position so that the pilot valve 62 moves somewhat in upward direction and pressure fluid is once more supplied to the top side of the piston 64, causing the latter to move downward. The described movements of the integral part of the regulator 17 continue until the desired steam temperature is obtained. The integral part of the regulator can be regulated by adjusting the throttle '71, the diiferential part can be controlled by adjustment of the throttle "/9 and the proportional part can be controlled by adjustment of/or exchanging the springs in the cylinders 56 and 73, or of the spring 53. The pressure in the conduit 16 acts through the pipe 72 also on the differential part of the regulator 17, which temporarily acts on the lever in the same manner as has been described in connection with the difierential part of the regulator 25 with respect to the lever 88. Increase of the pressure of the live steam causes, through the PID regulator 39, an increase of the pressure in the conduit 41 whereby the servo-piston 96 is moved to the right and the valve 5 in the feedwater pipe 4 is moved in closing direction.

The temperature measuring devices 23 and 15 adjust simultaneously with the adjustment of the valve 5 the combustion apparatus through the PD regulator 26 and the PID regulator 18. Increasing temperature causes closing of the regulating devices 11 and 12 for reducing the heat output of the combustion apparatus.

In the system shown in FIG. 3 the output of the PID regulator 17 is conducted through conduit 19 to the input side of the PD regulator 25 whereby the signals arriving through the conduit 19 adjust the set point of the PD regulator 25. Likewise, the output of the PID regulator 18 is conducted to the input side of the PD regulator 26. In lieu of the summing device 35 of FIG. 1 a combustion apparatus control device 107 is provided in the example shown in FIG. 3. The signals. produced by the regulators 26 and 38 are combined in the device 33 and are conducted through a conduit 34 to the control device or regulator 107 for primarily acting, for example, upon the fuel supply whereas the air supply is secondarily adjusted by the regulator 107 so that the flue gas has a predetermined CO content. The latter is measured by the device 43 and the corresponding signal is conducted to the regulator 107 through the conduit 42.

In the example shown in FIG. 3 a steam flow rate measuring device 44 is provided in lieu of the pressure responsive device of FIG I and produces signals which are conducted through conduits 45 and 45' to the regulators 38 and 39. If the steam generator is operated at constant load and if, for any reason, the rate of flow of steam in the steam main 7 increases, the output of the combustion apparatus and the feedwater supply are reduced. If the steam generator is operated at variable load, signals corresponding to the desired load at any moment are conducted through a conduit 47 to a summation device 46, the output signal of the device 46 being conducted through the conduit 45' and the regulators 38 and 39 to the devices for regulating the combustion apparatus 8 and the feed valve 5.

In the example shown in FIG. 4 the steam generator is like the steam generators shown in FIGS. 1 and 3. A turbine 104 is connected to the steam main 7 through a valve 103, the turbine driving an electric generator 105. In contradistinction to the example shown in FIG. 1 the pressure responsive device 36 acts through a PID regulator 102 on the valve 103 for controlling the admission pressure to the turbine. The regulator 102 receives a set point setting signal through a conduit 101 from an output control device 100. The latter is conventional and is provided with a plurality of cams mounted on a common shaft which cams adjust'the pressure in pressure fluid conduits controlled by the device 100. The output control device 100 may contain apparatus for transforming the time characteristic of the produced signals. Such device, for example, is shown in the pending patent application Serial No. 798,790, now Patent No. 3,086,503 granted April 23, 1963. In addition to the set point control signal for the regulator 102 the device 100 also produces a signal which is conducted through the conduit 37 to the PID regulators 38 and 39 which control the combustion apparatus and the feedwater supply, respectively. The device 100 also produces a signal which is conducted through a conduit 106 to the combustion apparatus regulator 107 for controlling the relation between the fuel supply and the combustion air supply. This signal corresponds with the necessary air excess which is a function of the load of the steam generator. In general the necessary air excess is smaller at higher load than at lower load. The input of the regulator 107 is connected through the signal conduit 34 to the summing device 33 which receives temperature signals from the temperature measuring devices 15 and 23 through the regulators 18 and 26, respectively, and which receives a set point signal through the regulator 38. There is no CO content control of the flue gases. The output control device 190 is manually operated according to the indications of a frequency measuring device connected to the generator-195. The device 104) may also be controlled automatically in response to the speed of the turbine 104 as indicated by a dotted line in FIG. 4.

The temperature control of the steam can be further improved by providing coolant injection at the end of the final superheater as shown in FIGS. to 7.

In the example shown in FIG. 5 the superheater 3 of FIG. 1 is divided into two serially arranged sections 3 and 3. The. temperature measuring device is connected downstream of the superheater section 3 and is connected through signal conduits 16 to the PID regulator 17 and to the PID regulator 18 as in the example shown in FIG. 1. Downstream of the final superheater section 3' an additional temperature measuring device 110 is provided which is responsive to the temperature t at the 7 end of the superheater section 3 and which is connected through a signal conduit 111 to a PID regulator 112. The latter acts on a valve 113 in a pipe 114 which terminates in the steam pipe connecting the superheater sections 3 and 3 and which supplies liquid or vaporous coolant to the steam. A rate of fiow measuring device 115 is interposed in the conduit 114 and produces signals conducted through a conduit 116 to an integral regulator 117 whose output is conducted through a signal conduit 118 to a summing device 119 which is interposed between the temperature measuring device 15 and the fork of the pipe 16 wherefrom one branch 16 leads to the regulator 17 and the other branch leads to the PID regulator 18 shown in FIG. 1.

The live steam temperature is measured by the device 110 and coolant injection is controlled by the regulator 112 which actuates the valve 113 to increase coolant injection upon increase of the temperature of the live steam above a predetermined temperature and vice versa. The flow rate of the coolant is measured by the device 115 and, upon considerable duration of the deviation of the coolant flow rate from the desired coolant flow rate, the set points of the regulators 17 and 18- are changed by the integral regulator 117 so that the deviations are taken care of already at the inlet of and within the steam generator by changing the feedwater supply and the output of the combustion apparatus. In this way, in the steady state the rate of flow of coolant is returned to the desired rate. The necessary range of operation of the 5?. injection valve 113 is therefore small and the amount of coolant injected is also small. The set point signal corresponding to the desired coolant fiow rate is supplied to the regulator 117 through a conduit 108.

In the arrangement according to FIG. 6 the temperature measuring device 15 is placed downstream of the final superheater section 3' and measures the temperature of the live steam. A signal conduit branches from the signal conduit 16 and leads to a PD regulator 126 which operates a valve 127 in a coolant injection pipe 128. The device 15 provides signals not only for the regulators 17 and 18 but also for the regulator 126 which temporarily adjusts the coolant injection accordingly. The arrangement shown in FIG. 6 is somewhat simpler than the arrangement shown in FIG. 5.

In the example shown in FIG. 7 the temperature measuring device 15 is arranged downstream of the final superheater section 3', as in FIG. 6, and is connected through a conduit 130 and a PID regulator 131 to a valve 132 interposed in a coolant injection pipe 133. In contradistinction to the previously described example, the device 15 is not directly connected to the PID regulators 17 and 18. In the example shown in FIG. 7 these regulators indirectly receive signals corresponding to the live steam temperature 1 the input conduit 16 receiving signals from a device 134 which is responsive to the rate of flow of the coolant through the pipe 133.

I claim: 1. A method of controlling a forced flow steam generator including a final superheater, and control means for the supply of feedwater, of fuel and of combustion air, comprising:

producing a first control signal having a PID characteristic and corresponding to the temperature of the steam superheated in the final superheater,

producing a second control signal having a PD characteristic and corresponding to the temperature of the operating medium of the steam generator upstream of the final superheater with respect to the flow of the operating medium through the steam generator, combining said first and said second control signals and controlling the feedwater supply in response to the combined first and second control signal for increasing the feedwater supply upon an increase of the combined signal, and conversely, producing a third control signal having a PID characteristic and corresponding to the temperature of the steam superheated in said final superheater,

producing a fourth control signal having a PD characteristic and corresponding to the temperature of the operating medium of the steam generator upstream of the final superheater, and

combining said third and said fourth signal and controlling the fuel and combustion air supply in response to the combined third and fourth signal for increasing the fuel supply and the air supply upon a decrease of the combined signal, and conversely.

2. A method of controlling a forced flow steam generator including a final superheater, and control means for the supply of feedwater, of fuel and of combustion air, comprising:

producing a first control signal having a PID characteristic and corresponding to the temperature of the steam superheated in the final superheater, producing a second control signal having a PD characteristic and corresponding to the temperature of the operating medium of the steam generator upstream of the final superheater with respect to the flow of the operating medium through the steam generator, adjusting said second control signal in response to said first control signal and controlling the feedwater supply in response to the adjusted second control signal for increasing the feedwater supply upon an increase of the adjusted signal, and conversely,

producing a third control signal having a PE) characteristic and corresponding to the temperature of the steam superheated in said final superheater,

producing a fourth control signal having a PD characteristic and corresponding to the temperature of the operating medium of the steam generator upstream of the final superheater, and

adjusting said fourth control signal in response to said third control signal and controlling the fuel and combustion air supply in response to the adjusted fourth signal for increasing the fuel supply and the air supply upon a decrease of the adjusted signal, and conversely.

3. Method as defined in claim 1 including producing signals having a proportional-integral-ditferentia1 characteristic and corresponding to the desired output of the steam generator, and individually combining said last mentioned signals with said combined first and second control signals and with said combined third and fourth control signals for decreasing the feedwater, fuel and combustion air supply upon a desired decrease of the output of the steam generator, and conversely.

4. Method as defined in claim 1 including producing a signal corresponding to the percentile CO content of the gas resulting from the combustion of the fuel, and supplementally controlling the air supply in response to said last mentioned signal for decreasing the air supply upon a reduction of the percentile CO content of the combustion gas, and conversely.

5. A method as defined in claim 1 including producing a fifth control signal corresponding to the temperature of the steam superheated in the final superheater, injecting a coolant into the steam produced in the steam generator, and controlling the rate of flow of the coolant inc jected into the steam in response to said fifth control signal for increasing coolant injection upon an increase of the temperature of the steam superheated in said, final superheater, and conversely.

6. A method according to claim 5 wherein said first and third control signals correspond to the rate of flow of the coolant injected into the steam instead of corresponding to the temperature of the steam superheated in the final super-heater.

7. A system for controlling a forced flow steam generator having a feedwater supply pipe, a feedW-ater control valve interposed in said supply pipe, tubular heating means including a plurality of heating sections and a final superheater ararnged in series relation with respect to the flow of the operating medium of the steam generator and connected to said feedwater supply pipe to receive feedwater therefrom, and a combustion apparatus associated with said heating means for supplying heat (thereto and including fuel supply control means and combustion air supply control means, said system comprising:

a first temperature sensitive device connected to said final superheater and including means for producing a first control signal corresponding to the temperature of the steam superheated in said final superheater,

a second temperature sensitive device connected to said heating means upstream of the point of connection of said first temperature sensitive device with respect to the flow of the operating medium through said heating means and including means for producing a second control signal corresponding to the temperature at the point of connection of said temperature sensitive device to said heating means,

means for combining said first and second control sigrials,

said combining means being operatively connected to said feedwater control valve for operating said con trol valve in opening direction upon an increase of the combined signal, and conversely,

said first temperature sensitive device including means for producing a third control signal corresponding to the temperature of the steam superheated in said final superheater,

said second temperature sensitive device including means for producing a fourth control signal corresponding to the temperature at the point of connection of said second temperature sensitive device to said heating means, means for combining said third and fourth control signals, said last mentioned combining means being operatively connected to said fuel supply and combustion air supply control means for controlling the fuel and combustion air supply in response to the combined third and fourth signal for increasing fuel and combustion air supply upon a decrease of the last mentioned combined signal, and conversely,

8. responsive to the pressureof the steam leaving the final superheater and including means for producing a control signal corresponding to said steam pressure, means connected to said last mentioned control signal producing means and to said feedwater control valve for supplementally operating said control valve in the closing direction upon an increase of the-steam pressure above a predetermined value, and conversely, and means connected'to said means for producing a control signal corresponding to said steam pressure and connected to said fuelsupply and combustion air supply control means for supplementally operating the latter in response to the last mentioned control signal for decreasing the fuel supply and the combustion air supply upon an increase a predetermined value, and conversely.

' References Cited by the Examiner UNITED STATES PATENTS 1,975,086 10/34 Dickey 122-451 2,623,698 12/52 Dickey 23624.5 2,526,898 10/50 Powell et al. 122-479 2,984,984 5/61 Dickey "122-479 2,985,153- 5/61 Dickey 122- 479 3,096,744 7/63 Profos 122 479 FOREIGN PATENTS 1,221,229 1/60 France.

1,244,634 9/60 France.

ROBERTA. OLEARY, Primary Examiner.

FREDERICK L. MATTESON, In, MEYER PERLIN,

, Examiners.

A system as defined in claim 7 including means 

1. A METHOD FO CONTROLLING A FORCED FLOW STEAM GENERATOR INCLUDING FINAL SUPERHEATER, AND CONTROL MEANS FOR THE SUPPLY OF FEEDWATER, OF FUEL AND OF COMBUSTION AIR, COMPRISING: PRODUCING A FIRST CONTROL SIGNAL HAVING A PID CHARACTERISTIC END CORRSPONDING TO THE TEMPERATURE OF THE STEAM SUPERHEATED IN THE FINAL SUPERHEATER, PRODUCING A SECOND CONTROL SIGNAL HAVING A PD CHARACTERISTIC AND CORRESPONDING TO THE TEMPERATURE OF THE OPERATING MEDIUM OF THE STEAM GENERATOR UPSTREAM OF THE FINAL SUPERHEATER WITH RESPECT TO THE FLOW OF THE OPERATING MEDIUM THROUGH THE STEAM GENERATOR, COMBINING SAID FIRST AND SAID SECOND CONTROL SIGNALS AND CONTROLLING THE FEEDWATER SUPPLY IN RESPONSE TO THE COMBINED FIRST AND SECOND CONTROL SIGNAL FOR INCREASING THE FEEDWATER SUPPLY UPON AN INCREASE OF THE COMBINED SIGNAL, AND CONVERSELY, PRODUCING A THIRD CONTROL SIGNAL HAVING A PID CHARACTERISTIC AND CORRESPONSING TO THE TEMPERATURE OF THE STEAM SUPERHEATED IN SAID FINAL SUPERHEATER, PRODUCING A FOURTH CONTROL SIGNAL HAVING A PD CHARACTERISTIC AND CORRESPONDING TO THE TEMPERATURE OF THE OPERATING MEDIUM OF THE STEAM GENERATOR UPSTREAM OF THE FINAL SUPERHEAT, AND COMBINING SAID THIRD AND SAID FOURTH SIGNAL AND CONTROLLING THE FUEL AND COMBUSTION AIR SUPPLY IN RESPONSE TO THE COMBINED THIRD AND FOURTH SIGNAL FOR INCREASING THE FUEL SUPPLY AND THE AIR SUPPLY UPON A DECREASE OF THE COMBINED SIGNAL, AND CONVERSELY. 